Electrical power distribution systems select power from one or more sources and distribute power to one or more loads. Each source may be a primary power source or an output of another part of the whole distribution system. Similarly, each load may be a power consuming device (e.g., a heater) or a power input to a stage of the distribution system itself. Thus, the system as whole may include one or more chains of power management devices.
In typical electrical power distribution systems, an electro-mechanical contactor (or relay) is commonly employed as a commutation device. Such a contactor typically has a low resistance to power flow when closed and a high resistance when opened. The latter state effectively prevents current from flowing from a source to a load.
A contactor can have undesirable characteristics, including at least 10 ms of switching time when changing states. Another adverse operational characteristic is when the contactor may fail to open when the current flow is very high, a failure mode that may occur if the contactor connects a low impedance source to a short circuit load.
High current power switching semiconductor devices have been used to mitigate the above-noted issues. When used in lieu of a contactor, these semiconductor devices form what is termed a solid state power controller (SSPC). An SSPC-based commutation device can open or close much faster than its contactor-based counterpart. Moreover, because the rate of change of the current is limited by the intrinsic inductance of the wiring to the load, an SSPC-based commutation device can open to remove power at a convenient and safe overload level before exceeding a maximum current transient level.
There are a number of component technologies (e.g., metal-oxide semiconductor field effect transistor (MOSFET), thyristor, triac, insulated gate bipolar transistor (IGBT)) that can be used for the power switching component(s) of an SSPC, but MOSFETs are typically preferred for low voltage applications (e.g., for voltages lower than 600V) because their voltage drop, and hence power loss, can be made smaller than what is achievable with the alternative component types.
Some configurations of unidirectional MOSFET-based SSPCs can only control current in one direction because a MOSFET includes an intrinsic diode that conducts when the potential difference across the device (from source to drain) is reversed in polarity. Bidirectional current flow control can be achieved by connecting two MOSFETs in series in a back-to-back fashion, so that one of the MOSFETs is able to block the current flow in one direction, while the other MOSFET is able to block the current flow in the opposite direction. However when current flow is enabled, the path impedance will be twice that of the simpler equivalent unidirectional device.
The path impedance can be reduced, and consequently the power loss can be reduced, by paralleling two or more MOSFETs. Moreover, paralleling two or more MOSFETs also enables the total current flow capacity to be increased. As such, replacing a single unidirectional SSPC with a bidirectional SSPC exhibiting (approximately) the same total power loss requires quadrupling the number of MOSFETs, assuming the majority of the power loss is due to the MOSFET(s), which is typically the case in optimal designs. Unfortunately, the quadrupling requirement for bidirectional control results in a high component count for high current SSPCs, which adversely affects the SSPC's reliability.